Electromagnetic Acoustic Transducers (EMATs) are electrical devices that can transmit and receive ultrasonic sound waves in an electrically conducting material without requiring contact of the probe with the material being inspected. EMATs are typically composed of arrays of electrical conductors, commonly referred to as coils and magnets, which can either be permanent magnets or electromagnets. When the electrical conductors comprising the arrays are energized by an alternating electrical current and placed in close proximity to an electrically conducting material, eddy currents are induced in the material. When these eddy currents are induced in the presence of a magnetic field, forces known as Lorentz forces are applied to the transient electrons of the induced eddy currents. A Lorentz force (F) at a point in the material is described mathematically by the cross product of the magnetic flux density (B), the induced eddy current density (J) and the relationship therebetween is shown in the following equation.F=J×B
The sum of these Lorentz forces produce an acoustic stress wave having the same frequency with respect to time as the induced eddy currents. The various types and modes of the waves which can be generated by EMATs are determined primarily by the configuration of the magnet, the configuration of the electrical conductors and their physical position with respect to each other. Guided waves such as the well known Lamb wave modes are easily generated in metal plates and cylinders by EMATs. These guided waves are generated near the surface of a section of the plate which is in close proximity to the EMAT and are transmitted from the EMAT in at least on direction within the material. Guided waves traveling under an EMAT are detected by the reverse process by causing the magnetic field under a conductor to change at the same frequency as the acoustic wave. As this alternating field is coupled to the electrical conductors of the EMAT coil, a signal voltage will be detected at the terminals of the coil as the acoustic wave travels under the EMAT.
EMATs offer several advantages when compared to piezoelectric transducers. EMATs do not require any fluid coupling for one and the acoustic waves that are generated are generated immediately below the surface of the material being tested unlike piezoelectric transducers in which the sound is produced in the probe and transferred to the material through a coupling medium such as oil or water. The latter characteristic provides substantially greater accuracy, reliability and repeatability for applications in which the test material is contaminated, rough, hot or moving at high speeds relative to the transducer. As EMAT fabrication can be very precise, components such as a sensor coil and/or a magnet or, even the entire EMAT, can be interchanged with little or no variation in acoustic beam shape and signal response to defects and/or to the material characteristics being detected or measured. Another advantage of the basic EMAT is its inherent simplicity of construction provides an almost unlimited variety of designs to implement shaping, steering and focusing acoustic beams to achieve the desired effect.
A further advantage of EMATs is their ability to generate guided waves in uniform metal components such as rods, plates and pipe. There are two basic types of guided waves, Lamb waves and horizontally polarized shear waves. Lamb waves are produced by the interaction of alternating magnetic fields with relatively constant magnetic fields at the surface of a metal component. The horizontally polarized shear waves are usually generated in ferromagnetic materials such as carbon steel and steel alloys which have the property of magnetostriction but can be generated through the interaction of magnetic fields which alternate in space but are constant in time with magnetic fields which are constant in space but alternate in time. The focus of this patent is the generation of Lamb waves in ferromagnetic and nonferromagnetic metal components which have a central axis symmetry.